As shown in, for example, Japanese Laid-Open Patent Publication No. 53-126817 (HO4N 5/4), a conventional television receiver is so arranged that a direct current restorer having an arrangement as substantially shown in FIG. 6, shifts the direct current level of a video signal such as a television signal, a three-primary-color signal or the like, thereby to maintain constant the pedestal level of the video signal.
More specifically, a video signal shown in FIG. 7 entered to an input terminal 1, is entered to the base of a transistor 5 of a comparator 4 through a transistor 2 and a resistance 3. A pedestal level setting reference signal having a voltage set by a variable resistance 7 is entered to the base of a transistor 6 forming a differential pair together with the transistor 5.
A constant-current source 8 in a common emitter circuit of the transistors 5, 6 is turned ON for every pedestal period, based on a clamp pulse of a video signal. The comparator 4 compares the pedestal level of the video signal with the level of the reference signal. According to difference in level between both signals, a transistor 9 is turned ON and a capacitor 10 is charged with a voltage corresponding to the difference in level between both signals. Thus, the difference in level is held.
Based on the charging voltage of the capacitor 10, a transistor 11 is turned ON. According to a resistance value across the collector and the emitter of the transistor 11, the video signal is shifted in direct current level. The video signal of which pedestal level is clamped as constant, is supplied to an output terminal 12.
In FIG. 6, a power terminal and a bias resistance are generally designated by reference numerals 13 and 14, respectively.
When displaying an image on a CRT, the variable resistance 7 is changed to adjust the pedestal level of the video signal.
For a liquid crystal television receiver or the like for displaying an image on a liquid crystal panel, the liquid crystal panel is generally driven by an alternating current. Accordingly, the video signal entered into the direct current restorer is inverted for every line or for every field.
In the direct current restorer applied to a liquid crystal panel drive circuit in a liquid crystal television receiver or the like, it is required that, based on the video signal of which polarity is inverted as shown in, for example, FIG. 8, the level of the reference signal is switched synchronously with such polarity inversion.
The Japanese Laid-Open Patent Publication No. 55-33365 (HO4N 5/16) discloses a direct current restorer comprising a comparator for comparing a difference in level between an input video signal and a reference signal, a sample-hold circuit for holding an output signal from the comparator, and an adder for adding an output signal from the sample-hold circuit, to the input video signal.
Also in the direct current restorer disclosed by this Publication, the direct current level of the video signal is shifted in a manner similar to that in the direct current restorer in FIG. 6.
The Japanese Laid-Open Patent Publication No. 60-124184 (HO4N 5/66) discloses a direct current restorer in which an alternating current output of an input video signal is superposed on a direct current passing through a variable resistance, thereby to maintain constant the pedestal level of the input video signal.
The conventional direct current restorer in FIG. 6 presents the problem outlined below. That is, the control period of the transistor 9 is limited to the pedestal period and the charging voltage of the capacitor 10 is gradually changed. Accordingly, even though the reference signal level is changed according to the variation of the set value of the pedestal level, the polarity inversion of the video signal and the like, the variation of the shifted amount of the direct current level does not quickly follow the level variation of the reference signal. Therefore, the control of the pedestal level of the video signal at the output terminal 12 is delayed, failing to achieve a stable direct current restoration.
In particular, when the direct current restorer in FIG. 6 is applied to a liquid crystal panel drive circuit, the polarity of the input video signal is inverted in a short period of time for every line or for every field, and the level of the reference signal varies in a relatively considerable amount due to the polarity inversion. Accordingly, the pedestal level cannot be adjusted in a stable manner to deteriorate the display driving characteristics and the like of the liquid crystal panel.
The direct current restorer disclosed by the Japanese Laid-Open Patent Publication No. 55-33365 above-mentioned also presents a problem similar to that above-mentioned.
The direct current restorer disclosed by the Japanese Laid-Open Patent Publication No. 60-124184 above-mentioned, cannot be disadvantageously used when the polarity of the input video signal is periodically inverted.